In the past twenty years, a great deal of emphasis in the field of immunochemistry has been placed on the development of new and improved techniques for immunoassay. Immunoassay relies, in principle, on the natural reactions of the body's immune system to the presence of foreign substances introduced into the body. The immune system is provoked by these foreign materials, for example, infectious organisms such as bacteria or viruses, to produce antibodies which react specifically with the foreign substance (or antigen) and which, if effective, aid in the elimination of the organisms from the body. The production of antibodies, of course, is not limited to the presence of infectious microorganisms but is also observed in response to many materials which are not normally found in circulation in the body.
The use of immunoassay techniques for diagnostic testing, that is detection of antigen or antibody, has recently become very widespread and now is so frequently employed in both research and clinical environments that it may be considered commonplace.
The relative specificity of antibody for a particular antigen has provided the basis for highly specific and accurate diagnostic testing for various physiological conditions such as infectious diseases, pregnancy and presence of drugs in the body. In practice, the test operates by exposing a test sample suspected of containing a particular antigen such as a bacterium or antibody to a particular microorganism such as the AIDS virus to a detectably labelled corresponding "immunological partner," i.e., the complementary antibody or antigen. The specificity of the antigen-antibody reaction is thus exploited in such well-known immunodiagnostic techniques as precipitation reactions, immunodiffusion, agglutination or hemolysis of antigen coated red cells, bacteria or latex particles, complement fixation, fluorescent immunoassays, immunoelectrophoresis, radioimmunoassay (RIA), and enzyme immunoassay (EIA), including enzyme-linked immunosorbent assay (ELISA). Two techniques of immunoassay, RIA and EIA, have become particularly popular because of their generally superior sensitivity, and the greater safety involved in EIA.
Many of these assays are sensitive and specific, but all are limited by one or more of the following: requirements of expensive laboratory equipment (e.g. gamma counter, fluorescence microscope, high speed centrifuge, spectrophotometer, etc.), a technician trained in the operation of these instruments, and reagents which present some hazard or require refrigeration. In addition, some of the assays are relatively insensitive, not quantitative and require several hours or even days to complete.
It would be highly desirable, therefore, to provide a cost effective, simple and sensitive diagnostic immunoassay which can provide an unequivocal positive and negative end point within a matter of minutes, which assay can be used quantitatively as well as qualitatively, that requires minimal laboratory equipment and that the components of the assay can be made shelf stable easily.